1. Field of the Invention
The present invention relates to stimulation of underground wells by injection of hydrocarbon-based fracturing fluids, and more particularly to gellation of such fluids. 2. Description of the Prior Art
In view of the scarcity and value of crude oil, crude oil producers are continually searching for methods and products for improved "well stimulation" to maximize recovery of crude oil from underground wells. One of the problems encountered in attempting to maximize oil recovery by well stimulation is oil entrapment. Oil wells often contain large amounts of crude oil entrapped in pockets in underground rock formations. Such entrapped oil does not tend to flow readily to the well bore and so from an economical standpoint, such entrapped oil generally is not recoverable under such conditions.
Thus, the art of well stimulation commonly employs a technique called "fracturing" to enhance the recovery of oil and gas from subterranean formations. Normally, fracturing involves the injection of viscosified aqueous or hydrocarbon fluids, which usually contain a proppant material, such as sand, into a well bore at such a rate and pressure as to exceed the formation stresses, thereby causing rock fatigue and inducing new fractures in the formation. Fractures are natural or induced cracks or channels in the formation matrix. After the exerted injection pressure has been relieved, fracture closure will occur and the induced cracks will be "propped" open by the proppant material. A more conductive channel is thus provided to allow the oil or gas to flow to the well bore after the injection pressure is relieved.
Viscous fluids possess several properties which are favorable to fracturing applications. For example, the fluid viscosity is proportionally related to the created fracture volume and fracture width. Higher fluid viscosities, therefore, will generate larger fracture volumes and fracture widths. Viscous fluids further serve as efficient proppant transporting media necessary to place a proppant into the opened fracture. In other words, proppants tend to settle out of low viscosity fluids before or soon after injection into a well, but to remain suspended in higher viscosity fluids. Therefore, fluids of higher viscosity more efficiently carry the proppant into fractures. Generally, it has been found that a viscosity of at least about 100 cps at 170 sec..sup.-1 is necessary to carry sand effectively.
The geology and geochemistry of various formations dictate the nature of the fracturing fluid, i.e., whether neutral or strongly acidic aqueous fluids or hydrocarbon fluids should be used.
Many wells are water-sensitive. For example, oil deposits are sometimes found in sandstone formations containing clay which, when exposed to water, tends to swell, thereby reducing the flow of oil. Accordingly, gelled hydrocarbons are frequently used as fracturing fluids. Hydrocarbon-based fracturing fluids typically are gelled by a phosphate/aluminum gelling system as described, for example, in U.S. Pat. Nos. 3,494,949, 3,575,859, 4,104,173 and others. According to this method, a phosphate composition, a liquid hydrocarbon such as crude oil, and an activator, particularly sodium aluminate, are mixed together. The phosphate compositions suitable for such applications are well known in the art and generally comprise an organic phosphate of the general formula ##STR2## wherein R is an alkyl group containing one to about eight carbon atoms and R' is an alkyl group or an alkenyl group containing about six to about eighteen carbon atoms, or a mixture thereof. Particular species typical of such phosphates are identified in the above-noted patents. Blending and circulation of the resulting mixture are then maintained for between about 1 and about 24 hours, until the viscosity of the mixture reaches a desired level. Viscosification is understood to take place as a result of the formation of phophate/aluminum complexes as aluminum ions from the activator, generally the aluminum ions from sodium aluminate, replace the acidic hydrogens of the phosphate.
In view of the advantages related to hydrocarbon-based fracturing fluids of high viscosity, the oil production industry is continually searching for hydrocarbon-based fracturing fluids of ever higher viscosities, and for efficient, low cost methods for producing such higher viscosities. Moreover, conventional hydrocarbon-based fracturing fluids tend to lose viscosity at higher temperatures, i.e., about 250.degree. F. or more, often found in wells. Accordingly, hydrocarbon-based fracturing fluids which maintain high viscosity at high temperatures are needed.
Another problem commonly encountered is that a period of from about 1 to about 24 hours after thorough mixing of gelling agents is required for complete or maximum gellation of the hydrocarbon-based fracturing fluid. Thus, the gelling agents must be added long before the fluid is to be injected into the well, thereby constraining or interfering with preparation or application procedures; and expensive labor hours can be wasted.
Therefore a quick, simple and low cost method for increasing the viscosity of hydrocarbon-based fracturing fluids and a hydrocarbon-based fracturing fluid which maintains its viscosity at high well temperatures are desired.